Modern analytical techniques

Question 1

How can bonds vibrate?

Answer 1

1. Bonds can vibrate by stretching. This is regarded as a high energy vibration.
2. Bonds can vibrate by bending. This is regarded as a low energy vibration.

Question 2

Why do bonds vibrate?

Answer 2

Bonds vibrate when they absorb energy. When they do, they absorb energy from parts of the EM spectrum that has the same frequency as the vibrations, which is mainly in the infrared spectrum.

Question 3

What determines the frequency of vibrations?

Answer 3

1. Bond strength.
2. Bond length.
3. Mass of atoms involved in bonding.

Question 4

What are the features of an infrared spectrum?

Answer 4

• The frequency of the part of the infrared spectrum is represented by wavenumber along the x-axis.
• The % transmittance is shown along the y-axis. When a frequency of infrared radiation is absorbed by a molecule, the spectrum troughs at that frequency.
• The trough is called a peak.

Question 5

What characterises the IR spectrum of an alcohol?

Answer 5

There is peak at wavenumber 3200-3550/cm representing the alcohol O-H group.

Question 6

What characterises the IR spectrum of a ketone/aldehyde?

Answer 6

There is a peak at wavenumber 1620-1750/cm representing the C=O bond.

Question 7

What characterises the IR spectrum of a carboxylic acid?

Answer 7

1. There is a peak at wavenumber 2500-3300/cm representing the carboxyl O-H group.
2. There is a peak at wavenumber 1620-1750/cm representing the C=O bond.

Question 8

What are the limitations of of IR spectroscopy?

Answer 8

IR spectroscopy is unable to distinguish aldehydes from ketones or alkanes from alkenes.

Question 9

What are the uses of IR spectroscopy?

Answer 9

1. Measuring how unsaturated a polymer is by detecting double bonds.
2. Quality control in perfume manufacturing.
3. Used in drug analysis, especially breath analysers testing for presence of alcohol.

Question 10

What are the uses of mass spectrometry in chemistry?

Answer 10

• To identify an unknown compound.
• To determine the relative abundance of isotopes.
• To find out more about the structure and chemical properties of a compound.

Question 11

How is mass spectrometry used to determine relative isotopic abundance?

Answer 11

A pure sample of an element is run through the mass spectrometer and the percentage abundances are displayed on the spectrum.

Question 12

What are the features of a mass spectrum?

Answer 12

• The mass to charge (m/z) ratio is displayed along the x-axis of the graph. This is because sometimes a species may be given a bigger than +1 charge.
• The percentage abundance is displayed on the y-axis of the graph. Data is discrete, so bars are used to display data.

Question 13

What is the molecular (M+) ion?

Answer 13

The positive ion formed in mass spectrometry when a molecule loses an electron.

Question 14

Why are we able to determine the molecular mass of compounds?

Answer 14

• The mass of the electron lost from the molecular ion is said to be negligible, thus the molecular ion mass is equal to the molecular mass of a compound.
• The molecular ion mass can be detected by a mass spectrometer.

Question 15

What is a fragmentation?

Answer 15

The process in mass spectrometry that causes a positive ion to split into pieces, one of which is a positive ion fragment.

Question 16

How is a fragment formed?

Answer 16

When the compound is bombarded with electrons and a molecular ion is formed, excess energy for ionisation can be transferred to the molecular ion, causing bonds to vibrate and weaken, eventually breaking.

Question 17

What are the limitations of fragmentation?

Answer 17

Identical compounds seem to fragment in similar ways, producing similar mass spectrums.

Question 18

What is the greenhouse effect?

Answer 18

The process in which the absorption and subsequent emission of infrared radiation by atmospheric gases warms the lower atmosphere and the planet’s surface.

Question 19

What are the 3 main greenhouse gases?

Answer 19

1. Water vapour.
2. Carbon dioxide.
3. Methane.

Question 20

How do greenhouse gases cause the greenhouse effect?

Answer 20

1. Greenhouse gas molecules absorb infrared radiation reflected from the surface of the earth.
2. The gas molecules vibrate in a number of different directions. Subsequent vibrations cause some infrared radiation to be emitted in all directions.
3. Some radiation is reabsorbed by other molecules and the process continues.
4. Some radiation goes back towards the earth’s surface where it gets absorbed.

Question 21

What factors determine a gas’ contribution to global warming?

Answer 21

1. Concentration of the gas in the atmosphere.

2. How good the gas is at absorbing infrared radiation